The invention deals generally with underground structures such as manholes, utility vaults, and pump stations, and more specifically with a method and apparatus to attach an impermeable continuous liner to prevent corrosion by the effects of sewer gas and to prevent fluid leaking into the underground structure.
Underground structures such as manholes serve to connect pipes, transfer sewage, and provide maintenance access. When they permit ground water to leak in, they contribute to unnecessary sewage treatment costs or damage the sewer lines or other utility services which pass through the underground structures. Furthermore, microorganisms that consume sewer gas form sulfuric acid, and this acid dissolves the underground structure walls which may lead to deterioration, collapse, service interruption, or accidents.
This deterioration is caused mainly because of the nature of the original structure. Underground structures such as manholes are essentially chambers in the ground, sometimes large vertical shafts, which extend to the depth at which sewer pipes or utility services are located. The older chambers are usually built of bricks or cement blocks, with the bricks or blocks assembled with mortar joints. These materials, and particularly the joints, deteriorate with time because of such factors as traffic loads, ground water, soil pressure, and septic gases. Even cast concrete underground structures can be damaged by such causes, particularly from the acids septic gases create and which attack most materials.
Once an underground structure is damaged and leaking, it is very difficult to repair it so that it is watertight and gas tight, and completely rebuilding it is costly and time consuming because it requires excavating all around the underground structure.
Several patents have been issued on a newer approach to repairing underground structure chambers. The technique involves attaching a liner to the inside wall surface of the underground structure chamber. As described in U.S. Pat. Nos. 5,490,744 and 5,265,981 by McNeil, the liner is typically a long fiberglass bag covered with an epoxy resin. This bag is lowered into the underground chamber, inflated by the use of a removable interior inflatable bladder until it presses against the inside walls of the underground chamber, and the resin is cured in place. The result is the formation of a new chamber which conforms to the original underground structure regardless of whether the chamber is a straight cylinder or it has an irregular shape. However, this type of additional internal chamber still has problems.
The structure of the McNeil liners, which have fiberglass and resins on the exposed surfaces, are themselves attacked by septic gases. This causes erosion of the exposed fiberglass layers which deteriorate over time and ultimately weaken the rehabilitation structure. Furthermore, at liner termination points such as junctions where the liner is joined to pipes and flow channels, gas infiltration leads to corrosion of the underground structure walls and destruction of the liner bond.
It would be very beneficial to have a underground structure liner which was chemically stable, allows gas-tight joints with pipe lining, and prevents fluid leakage into the underground chamber.